SHERMAN, Texas — Investigators say a blown tire on a charter bus that crashed and killed 15 people had been refitted with a new tread in violation of safety standards.

The National Transportation Safety Board also said late Friday that the driver was a 52-year-old who had a commercial license but whose medical certification had expired.

Authorities say the bus was operating illegally earlier in the day when the right front tire blew. The vehicle smashed into a guardrail and tipped over.

Officials say 12 people died at the scene and three others at Dallas-area hospitals.

The bus was carrying a Vietnamese Catholic group from Houston to an annual pilgrimage in Missouri.

The bus, with 55 people aboard, tipped over along the edge of the road at about 12:45 a.m., crushing one side of the vehicle and scattering luggage, clothes, a sandal and a blood-soaked pillow across the grass and pavement.

Passenger Leha Nguyen, 45, said people were dozing off aboard the bus when she heard a noise and screaming, and opened her eyes.

"Somebody was laying on my legs. A lady next to me, she had her arm crushed up. The lady who was on my left, a man was on top of her," she said at a hospital. She said nobody had been wearing seatbelts, and people were strewn all over. A television had fallen on one person.

"I think I'm the luckiest one out of most people," she said.

Most of the passengers were from the Vietnamese Martyrs Church and two other mostly Vietnamese congregations in Houston. They were on their way to Carthage, Mo., for an annual open-air festival honoring the Virgin Mary.

The Marian Days pilgrimage, begun in the late 1970s, attracts thousands of Catholics of Vietnamese descent and includes a large outdoor Mass each day, entertainment and camping at night.

"Please pray for us," said Holly Nguyen, a 38-year-old church member who was following behind the bus in a car but did not see the wreck. She anxiously awaited word of her father, who was on the bus when it ran off the road about 65 miles north of Dallas, close to the Oklahoma line. The bus operator, Iguala BusMex Inc. of Houston, had applied in June for a federal license to operate as a charter but was still awaiting approval, according to online records.

The company recently filed incorporation papers, listing the same owner and address as Angel Tours Inc., which was forced by federal regulators to take its vehicles out of interstate service June 23 after an unsatisfactory review, records show. Details of the review were not in the online records.

Neither entity is currently authorized to operate as a carrier in interstate commerce, said John H. Hill, administrator for the Federal Motor Carrier Safety Administration.

"We have requested law enforcement agencies to be alert for any buses being operated by Angel Tours or Iguala BusMex, since they are not authorized to operate legally," he said in a written statement. "If found on the road, we want law enforcement to immediately stop and place the vehicles out of service."

In a Houston building with a weathered Angel Tours plywood sign, a man declined to identify himself Friday or comment to The Associated Press about the wreck.

The tragedy was the nation's deadliest bus crash since 2004, when 15 people were killed in a wreck in Arkansas on their way to Mississippi's casinos. In 2005 near Dallas, 23 people were killed when a bus carrying nursing home residents away from Hurricane Rita caught fire while in bumper-to-bumper traffic.

The National Transportation Safety Board sent investigators to the scene of the Texas wreck.

"It appears a right tire on the tour bus had a blowout that contributed to the accident," said Sherman police Lt. Steve Ayers. The driver was reported in stable condition.

The Rev. Joseph Vu, a priest at the Vietnamese Martyrs Church and vicar for the 30,000 to 35,000 Vietnamese Catholics in the region, was not on the trip but arrived at a relief station set up for victims' families at a church in nearby Denison.

"I'm going to tell people we don't blame anybody," he said. "This happened like Katrina, like Challenger. What we can do is pray." He added: "God will comfort them. Tell people to keep trusting in God. Do not blame anybody. Do not ask why. Now we just help each other to get through this."

A sobbing Mary Nguyen, a member of the Vietnamese Martyrs Church for more than 10 years, learned that a close friend had died. "She was just a very good person," she said. "The church is like one big family here. We're very close. We stick together."

Organizers of the festival in Missouri said the victims would be remembered at Mass and at various conferences during the gathering.

The identities of the victims were not immediately released.